VARIATIONS IN TOTAI/;QUANTITY OF BLOOD 1059 



fluid would be to increase the mean systemic pressure, and therefore one 

 would expect to find a large increase both in arterial and venous systems. 

 But the organism prevents the rise on the arterial side by relaxing the 

 whole system of arterioles, so that the distribution of pressures is altered, 

 and the venous approximates more closely to the arterial pressure. This 

 arterial dilatation augments the velocity of the blood : it has been found that 

 the velocity may be accelerated to six or eight times the normal rate by 



Systol( 



Diastole 

 Seconds 



FIG. 487. Cardiometer tracing from dog's heart to show effect of increasing the 

 volume of circulating blood (hydrasmic plethora) on the total output and the 

 volume of the heart. Between the parts A and B 30 c.c. of warm normal salt 

 solution were injected intravenously, and between B and c 20 c.c. more. It will 

 be noticed that both the systolic and the diastolic volume are increased, i. e. 

 the heart is more distended during diastole, and does not contract to its 

 normal size in systole. The contraction volume, and therefore the output, 

 is very largely increased. (RoY.) 



injecting an amount of salt solution equivalent to 50 per cent, of the total 

 blood. 



The high venous pressure causes increased diastolic filling of the ventricles, 

 and therefore augments the strength of the beat. The frequency is also 

 generally raised if the vagi are intact in consequence of the greater 

 distension of the auricles. Thus the work of the heart is increased in three 

 ways, viz. by 



(1) Rise of arterial pressure. 



(2) Greater frequency of beat. 



(3) Increased output at each beat (Fig. 487). 



These series of changes result in the relief of the vascular system. The 



